Coal fired boilers for power plants and the like must comply with emission standards in the United States and in other countries throughout the world with regard to both particulate fly ash and with sulfur dioxide (SO.sub.2) emissions. In one prior art approach the problems are treated individually with the fly ash being collected for disposal and the SO.sub.2 being removed by the lime or limestone slurry in a scrubber or absorber. Such a system can be represented by the following flow chart. ##STR1##
In the above system the fly ash is generally collected by use of either an electrostatic precipitator or a filter bag house.
It is also known to utilize the fly ash, however, as a source of alkali for removal of SO.sub.2 from the flue gas, and in such case the system flow chart is as follows: ##STR2##
In the utilization of fly ash as a source of alkali, the fly ash is acid leached and in general the lower the pH of the leaching solution, the more alkali will be removed from the fly ash.
It is also known that to maximize the use of fly ash alkali the scrubber system should be operated at a pH of 4 or less.
The sulfur content as well as the ash and alkali content of coal varies from place to place over a considerable range. The use of fly ash as an alkali source depends upon the ratio of calcium and other cations to the amount of sulfur in the coal.
In a lime or limestone SO.sub.2 flue gas scrubbing system the slurry is recirculated through an absorption tower for contact with the flue gas to be cleaned. The SO.sub.2 is thus dissolved in the water phase of the scrubbing medium to produce sulfurous acid. In such a lime or limestone system calcium ions are produced by dissolution of calcium hydroxide or calcium carbonate which then reacts with the absorbed SO.sub.2 to produce an insoluble solid precipitate. In the utilization of collected fly ash, however, the cations to react with the SO.sub.2 can be provided all or in part by the acid leached fly ash rather than by the addition of lime or limestone to the scrubbing slurry.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof, which will be exemplified in the process hereinafter disclosed, and the scope of the invention will be indicated in the claims.